The total number of protons and neutrons in an atomic nucleus determines mass number. It is also called atomic mass number or nucleon number. The mass number is written either after the element name or as a superscript to the left of an element's symbol. Like in the element carbon, its most common isotope is carbon-12, or can be written as 12C, which has 6 protons and 6 neutrons.
Answer: Mass number, in nuclear physics, the sum of the numbers of protons and neutrons present in the nucleus of an atom.
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